Arrayed neutron detector with multi shielding allowing for discrimination between radiation types

ABSTRACT

Neutron detectors including one or more gamma shields over memory dies and methods of making the neutron detectors are provided. The neutron detectors can contain two or more memory dies, neutron-reactant layers over the two or more memory dies, and one or more gamma shields over at least a portion of or an entire of the two or more memory dies. By containing the gamma shield over the at least a portion of or an entire of the two or more memory dies, the neutron detector can detect and discriminate neutrons in the presence of gamma rays.

TECHNICAL FIELD

Described are neutron detectors with multi shielding allowing fordiscrimination between radiation types and methods of forming theneutron detectors.

BACKGROUND

Conventional neutron detectors generally include a sealed vesselcontaining a neutron sensitive gas, such as ³He or BF₃, and anelectrically charged wire having leads which extend outside of thevessel. In operation, incident neutrons react with the gas and producecharged particles. The charged particles change the electrical potentialof the wire. A measurement system coupled to the charged wire measuresthe electrical pulses and uses this information to indicate the presenceof neutrons. These types of neutrons detectors are usually undesirablybulky and are associated with poor sensitivity resulting from, forexample, electronic noise.

Attempts have been made to produce more portable neutron detectors usingsemiconductors. For example, ³He is diffused into a semiconductorsubstrate and used in the detection of neutrons. Neutrons react with the³He gas and produces hole-electron pairs in a depletion layer within thesemiconductor. The hole-electron pairs produce output electrical pulseswhich appear at the output terminals of the detector. The electricalpulses are utilized for detecting neutrons.

Semiconductor-based radiation detectors generally have a single-crystalsubstrate with a p-n junction or a Schottky junction. An inverse bias isapplied to the depletion layer. When radiation in the form of neutrons,gamma-rays, X-rays, electrons, protons, etc. are absorbed by thematerial, electron-hole pairs are created. These charges give rise to acurrent that is a measure of the intensity of the radiation fluxdetected by the detector.

SUMMARY

The following presents a simplified summary of the innovation in orderto provide a basic understanding of some aspects of the innovation. Thissummary is not an extensive overview of the innovation. It is intendedto neither identify key or critical elements of the innovation nordelineate the scope of the innovation. Its sole purpose is to presentsome concepts of the innovation in a simplified form as a prelude to themore detailed description that is presented later.

One aspect of the subject innovation provides neutron detectors. Theneutron detectors can discriminate neutrons in the presence of gammarays. The neutron detectors contain two or more memory dies containing aplurality of memory cells on a semiconductor substrate; neutron-reactantlayers over the two or more memory dies; and one or more gamma shieldsover at least a portion of or an entire of the two or more memory dies.By containing the gamma shield over the memory dies, the neutrondetector can detect and discriminate neutrons in the presence of gammarays.

Another aspect of the subject innovation provides methods of makingneutron detectors that can discriminate neutrons in the presence ofgamma rays. The methods include forming neutron-reactant layers over twoor more memory dies containing a plurality of memory cells and formingone or more gamma shields over at least a portion of or an entire of thetwo or more memory dies.

Yet another aspect of the subject innovation provides methods ofdetecting strength of a neutron field. The method includes providing aneutron detector containing two or more memory dies, neutron-reactantlayers over the two or more memory dies, and one or more gamma shieldsover at least a portion of or an entire of the two or more memory dies,the memory die containing a plurality of memory cell; reading a state ofeach of the memory cells of the memory dies to determine a number ofmemory cells changing from an initial state to a disturbed state; anddetermining the strength of the neutron field by using the number ofmemory cells having the disturbed state.

To the accomplishment of the foregoing and related ends, the innovation,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the innovation. These embodiments are indicative,however, of but a few of the various ways in which the principles of theinnovation may be employed. Other objects, advantages and novel featuresof the innovation will become apparent from the following detaileddescription of the innovation when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d are top views of exemplary neutron detectors containing anarray of multiple memory dies in accordance with one aspect of thesubject innovation.

FIG. 2 is a top view of a portion of an exemplary neutron detector inaccordance with one aspect of the subject innovation.

FIG. 3 a is an exemplary cross-sectional isometric illustration of amemory die that is not covered with a gamma shield in accordance withone aspect of the subject innovation.

FIG. 3 b is an exemplary cross-sectional isometric illustration of amemory die that is covered with a gamma shield in accordance with oneaspect of the subject innovation.

FIGS. 4-8 illustrate an exemplary method for forming a neutron detectorin accordance with one aspect of the subject innovation.

FIGS. 9-11 illustrate how the state of memory cells can change in thepresence of a neutron field in accordance with one aspect of the subjectinnovation.

FIG. 12 illustrates an exemplary methodology of forming a neutrondetector in accordance with one aspect of the subject innovation.

FIG. 13 illustrates an exemplary methodology of detecting strength of aneutron field in accordance with one aspect of the subject innovation.

DETAILED DESCRIPTION

Neutron detectors described herein can contain two or more memory dies,neutron-reactant layers over the two or more memory dies, and one ormore gamma shields over at least a portion of or an entire of the two ormore memory dies. The neutron detectors can employ semiconductor devicessuch as memory dies (e.g., flash memories). The memory dies can containa plurality of memory cells on a semiconductor substrate. Penetration ofneutrons and gamma rays into memory cells can change their state such asa logical 1 or 0. The gamma shields can facilitate discriminationbetween radiation types (e.g., neutron and gamma ray). The gamma shieldcovers at least a portion of or an entire of an array of multiple memorydies and substantially prevents penetration of gamma rays into thememory dies. As a result, the gamma shield on the memory dies canfacilitate to detect and discriminate neutrons in the presence of gammarays.

Because the neutron detectors contain the gamma shield, neutrons can bedetected efficiently. For example, by selectively shielding some ofmemory dies, flux and energy of incoming radiation at the shieldedportion and at the unshielded portion of the memory dies can becompared. The neutron detector can eliminate occurrences when gamma raysare triggering the neutron detector from the events when high-energyparticle (e.g., neutrons) causes excitation of the detector. As aresult, spurious readings caused by gamma rays can be eliminated,thereby allowing for more efficient neutron detection.

The neutron detector can employ any type of semiconductor device (e.g.,memory die) as long as the state of the semiconductor device is changedwhen particles associated with neutrons penetrate the semiconductordevice. An initial undisturbed state (e.g., logical 1) of asemiconductor device can be changed to a disturbed sate (e.g., logical0) when particles associated with neutrons penetrate the semiconductordevice. The neutron detector can employ memory cells such assingle-level memory cells, multi-level memory cells, single bit memorycells, dual bit memory cells, quad bit memory cells, or the like as amemory die. The dual bit memory is a relatively modern memory technologyand allows multiple bits to be stored in a single memory cell. The dualbit memory cell is essentially split into two identical (mirrored)parts, each of which is formulated for storing one of two independentbits. Each dual bit memory cell, like a traditional cell, has a gatewith a source and a drain. However, unlike a traditional stacked gatecell in which the source is always connected to an electrical source andthe drain is always connected to an electrical drain, respective dualbit memory cells can have the connections of the source and drainreversed during operation to permit storage of two bits. Other types ofmemory cells such as dynamic random access memory (DRAM) cells, staticrandom access memory (SRAM) cells, or charge coupled devices (CCD) canbe also employed.

The innovation is now described with reference to the drawings, whereinlike reference numerals are used to refer to like elements throughout.In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the subject innovation. It may be evident, however,that the innovation can be practiced without these specific details. Inother instances, well-known structures and devices are shown in blockdiagram form in order to facilitate describing the innovation.

Although the neutron detector can employ any type of memory die, theinnovation is hereinafter illustrated and described in the context of anexemplary semiconductor device having one or more memory arrayscontaining single bit memory cells arranged in a virtual ground typearray architecture. FIGS. 1 a-1 d illustrate top views of exemplaryneutron detectors containing two or more memory dies. Each memory diecan include M by N array cores. In FIG. 1 a, a neutron detector 100 acontains two or more memory dies (e.g., memory arrangements or memorydevices) 102 with a row 104 and a column 106. Each memory die 102 cancontain a plurality of memory cells (not shown). The memory cells areformed upon a semiconductor substrate (not shown). The memory dies 102contain neutron-reactant layers over the plurality of memory cells (notshown). The neutron-reactant layer can react with neutrons and emit oneor more particles capable of changing the state of memory cells. Forexample, when a neutron reacts with the neutron-reactant layercontaining ¹⁰Boron, a ⁷Lithium particle and a ⁴Alpha particles areemitted. Either of these particles can change the state of memory cells.

The neutron detector 100 a can further contain one or more shields overa portion of or an entire of the two or more memory dies 102. In FIG. 1a, a gamma shield 108 covers memory dies 102 at a peripheral portion ofthe neutron detector. A portion of the memory dies indicated by a dashedline illustrates that the portion is under the gamma shield 108. Aportion of the plurality of memory dies indicated by a solid lineillustrates that the portion is not covered by the gamma shield 108. Thegamma shield 108 substantially prevents penetration of gamma rays intothe memory dies 102 under the gamma shield. As a result, while thememory dies illustrated by the dashed line under the gamma shield candetect and discriminate neutrons against gamma rays, the memory diesillustrated by the solid line not covered by the gamma shield can detectboth neutrons and gamma rays.

Neutron detectors can contain any suitable shape of gamma shields over aportion of or an entire of two or more memory dies. In one embodiment,in FIG. 1 b, a gamma shield 108 is formed at a center of an array ofmultiple memory dies 102. In another embodiment, in FIG. 1 c, a gammashield 108 is formed at one side of an array of multiple memory dies102. In yet another embodiment, in FIG. 1 d, two triangular gammashields 108 are formed diagonally opposite each other at corners of anarray of multiple memory dies 102. In still yet another embodiment, ashape of the gamma shield is a triangle, square, rectangular, circle,oval, or the like (not shown).

One or more gamma shields 108 can be formed over any suitable number ofmemory dies 102. In one embodiment, one or more gamma shields are formedover about 5% of memory dies or more and about 95% of memory dies orless. In another embodiment, one or more gamma shields are formed overabout 10% of memory dies or more and about 90% of memory dies or less.In yet another embodiment, one or more gamma shields are formed overabout 20% of memory dies or more and about 80% of memory dies or less.

FIG. 2 illustrates a top view of an exemplary memory die (e.g., memorydevice) 200. The memory die 200 generally includes a semiconductorsubstrate (not shown) on which one or more memory arrangements (e.g.,high-density core regions) 202 and coupled to a controller 204.

The memory arrangement 202 typically includes a M by N memory array anda neutron-reactant layer over the memory array. The memory arrangement202 may include a gamma shield over the memory array, or the memoryarrangement 202 may not include a gamma shield. The memory arrangement202 includes individually addressable, substantially identical memorycells (e.g., single bit memory cells, dual bit memory cells, quad bitmemory cells, or the like). The memory arrangement can include anysuitable number of memory cells. For example, the memory arrangementincludes about 1-megabit memory arrays or more and about 4-megabitmemory arrays or less. Each of memory cells can store a state, such as alogical 1 or 0.

Advantageously, the controller 204 can be formed on the samesemiconductor substrate as the memory arrangement 202. This can allowfor a portable and compact neutron detector. For example, detectorshaving dimensions of about ¾″×¾″×¼′ is manufactured. The neutrondetectors can, for example, be worn on the wrist of a user, similar to awatch. In another embodiment, the controller 204 is formed on a separatesemiconductor substrate than the memory arrangement 202.

The controller 204 is generally coupled to the memory arrangement 202using a row decoder 206 and a column decoder 208. By providing a rowaddress and a column address to the row decoder 206 and the columndecoder 208, respectively, the controller 204 is able to read and writethe state of each memory cell in the memory arrangement 202.

In general, the controller 204 sets the state of each memory cell in thememory arrangement 202 and periodically reads the state of each memorycell to determine whether the state of the memory cell is changed. Forexample, the controller 204 sets the state of each memory cell to aninitial undisturbed state of logical 1 and read a state of each of thememory cells to determine a number of memory cells changing from aninitial undisturbed state to a disturbed state. Using this information,the controller 204 can determine the presence and strength of a neutronfield. The details of the controller and the process performed by acontroller to detect the presence of neutrons are not critical to thepractice of the methods. The details of the process of the controllercan be found in, for example, U.S. Pat. No. 6,075,261, entitled “NeutronDetecting Semiconductor Device,” issued Jun. 13, 2000, which is herebyincorporated by reference.

FIGS. 3 a and 3 b are cross-sectional isometric illustrations ofportions of an exemplary neutron detector as indicated in connectionwith FIG. 1. FIG. 3 a is a cross-sectional isometric illustration of aportion 300 a of a memory arrangement that is not covered with a gammashield. FIG. 3 b is a cross-sectional isometric illustration of aportion 300 b of a memory arrangement that is covered with a gammashield. The portions of the neutron detector 300 a, 300 b can containmemory cells 302 formed on a semiconductor substrate 304. The neutrondetector 300 a, 300 b can contain any suitable type of memory cells aslong as a state of the memory cell can be changed when particlesassociated with neutrons penetrate the memory cell. By way of example,FIGS. 3 a and 3 b illustrate portions of a neutron detector 300 a, 300 bincluding typical NAND-type flash memory cells 302. Although not shown,a neutron detector can include NOR-type flash memory cells or otherflash memory cells.

The memory cells 302 typically include bit lines (e.g., a source region306 and a drain region 308) and a channel region 310 in a substrate 304,and a stacked gate structure 312 overlying the channel region 310. Thesource region 306 and drain region 308 can contain an N-type of highimpurity concentration and are separated by a predetermined space of achannel region 310 which can be of P-type. The substrate 302 can be anN-type substrate. The bit line 306, 308 can contain an isolation region314 containing dielectric materials. Although not shown, a bit lineopening 316 between the stacked gate structures can contain a bit linedielectric such as oxides (e.g., silicon oxide, high temperature oxide(HTO), HDP oxide).

A stacked gate structure 312 typically includes a floating gate formedby a first polysilicon (poly I) layer and a control gate formed by asecond polysilicon (poly II) layer. A floating gate is isolated from acontrol gate by an interpoly dielectric layer and from channel region bya thin oxide layer which has a thickness of, for example, about 100angstroms. The thin oxide layer is commonly referred to as a tunneloxide. An interpoly dielectric layer can contain a multilayer insulatorsuch as an oxide-nitride-oxide (ONO) stack.

The neutron detector 300 a, 300 b can further contain neutron-reactantlayers 318 over the memory cells 312. The neutron-reactant layer 318 cancontain dielectric materials and neutron-reactant materials. Anysuitable neutron-reactant material can be employed in theneutron-reactant layer as long as the neutron-reactant material canreact with neutrons to emit one or more particles capable of changingthe state of memory cells 302. Examples of neutron-reactant materialsinclude ¹⁰Boron, ⁷Lithium, ²³⁵Uranium, or the like. As will be discussedfurther below, when a neutron reacts with ¹⁰Boron, for example, a⁷Lithium particle and a ⁴Alpha particle are emitted. Either of theseparticles can change the state of a memory cell 302.

Any suitable dielectric material can be employed in the neutron-reactantlayer 318. General examples of dielectric materials of theneutron-reactant layer 318 include silicon based dielectric materials,oxide dielectric materials, silicates, and low k materials. Examples ofsilicon based dielectric materials include silicon dioxide, and siliconoxynitride. Examples of silicates include fluorine doped silicon glass(FSG), tetraethylorthosilicate (TEOS),borophosphotetraethylorthosilicate (BPTEOS), phosphosilicate glass(PSG), BPSG, and other suitable spin-on glasses. Examples of low kmaterials include polyimides, fluorinated polyimides, polysilsequioxane,benzocyclobutene (BCB), poly(arylene ester), parylene F, parylene N, andamorphous polytetrafluoroethylene.

In one embodiment, the neutron-reactant layer 318 contains an oxide,such as SiO₂, doped with a relatively high concentration of ¹⁰Boron. Inanother embodiment, the neutron-reactant layer 318 contains aborophosphosilicate glass (BPSG) having a relatively high concentrationof ¹⁰Boron (e.g., ¹⁰BPSG). The concentration of ¹⁰Boron can be suitablyselected in consideration of the desired sensitivity of the neutrondetector 300 as well as in consideration of device reliability. In oneembodiment, the neutron-reactant layer 318 contains about 60 wt % of¹⁰Boron or more and about 90 wt % of ¹⁰Boron or less.

In naturally occurring Boron, typically, a concentration of ¹¹Boron isrelatively high as compared to a concentration of ¹⁰Boron isotope. Forexample, naturally occurring Boron typically includes 20 wt % of ¹⁰Boronisotope and 80 wt % ¹¹Boron isotope. In the neutron-reactant layer 318,suitable concentrations of ¹⁰Boron range from about 80 wt % to about 100wt % of the total Boron concentration. In some embodiments,concentrations of ¹⁰Boron range from about 95 wt % to about 100 wt % ofthe total Boron concentration.

Neutron-reactant layers 318 can be formed by any suitable technique. Forexample, a ¹⁰BPSG layer can be formed by BPSG deposition techniquesusing a source of Boron having a relatively high concentration of the¹⁰Boron isotope. The ¹⁰BPSG layer can be formed using, for example,spin-on techniques, chemical vapor deposition (CVD) techniques. CVDincludes pulsed plasma enhanced chemical vapor deposition (PECVD) andpyrolytic CVD as well as continuous PECVD. In one embodiment, a ¹⁰BPSGlayer is formed by forming a phosphosilicate glass (PSG) layer overmemory cells 302 and selectively implanting a relatively highconcentration of ¹⁰Boron into the PSG layer. In one embodiment, aconcentration of ¹⁰Boron ranging from about 80 wt % or more and about100 wt % or less of the total Boron concentration is implanted. Inanother embodiment, a concentration of ¹⁰Boron ranging from about 95 wt% or more and about 100 wt % or less of the total Boron concentration isimplanted. The details of the implanting techniques for selectivelyimplanting Boron isotopes into a PSG layer can be found in, for example,U.S. Pat. No. 5,913,131, entitled “Alternative Process for BPTEOS/BPSGLayer Formation,” issued Jun. 15, 1999, which is hereby incorporated byreference.

The portion 300 b of the neutron detector can further contain a gammashield (e.g., gamma discriminators) 320 over the plurality of memorycells 302. The gamma shield 320 covers the memory cells 302 tosubstantially prevent penetration of gamma rays into the covered portionof the memory cells but allow penetration of neutrons into the memorycells.

The gamma shield 320 can contain any suitable material as long as thematerial can substantially prevents penetration of gamma rays into thecovered portion but allow penetration of neutrons into the memory cells302. Examples of gamma shield materials include metals such as lead,bismuth, tungsten, steel, copper, brass, zinc, cobalt, alloys thereof,combinations thereof, or the like.

The gamma shield 320 can have any suitable thickness to facilitatedetecting neutrons and discriminating against gamma rays. The thicknessmay vary and is not critical to the subject innovation. The thickness ofthe gamma shield 320 may depend on, for example, gamma energy levelsinvolved, desirable gamma discrimination capability, desirabledimensions (e.g., thickness) of the neutron detector 300, theconfiguration and/or constituent of the gamma shield, the desiredimplementations, and/or the neutron detector 300 being fabricated. Inone embodiment, when a gamma shield is a lead (Pb) layer, a thickness ofthe lead gamma shield is about 1 mm or more and about 1,000 mm or less.In another embodiment, a thickness of a lead gamma shield is about 2 mmor more and about 500 mm or less. In yet another embodiment, a thicknessof a lead gamma shield is about 3 mm or more and about 400 mm or less.

Referring to FIGS. 4 to 8, one of many possible exemplary embodiments offorming a neutron detector is specifically illustrated. FIG. 4illustrates a cross sectional view of an intermediate state of portionsof memory dies of an exemplary neutron detector 400. In a portion of amemory die 400 a, a gamma shield is not formed over memory cells 402. Ina portion of another memory die 400 b, a gamma shield is formed overmemory cells 402 in subsequent processes. Although both portions 400 a,400 b are illustrated in FIGS. 4 to 8, they need not be formed at thesame time. In one embodiment, they are formed at the same time. Inanother embodiment, they are formed at different times separately.

In both portions 400 a, 400 b, memory cells 402 can be formed on asemiconductor substrate 404. The memory cell 402 is generally formedbetween isolation regions 406 on a semiconductor substrate 404 andincludes source/drain regions 408 and a gate structure 410. Forsimplicity of illustration in FIG. 4, two memory cells 402 are shown.However, the neutron detector 400 can have any suitable number of memorycells 402 in a memory die. For example, the memory die can have a M×Narray of memory cells 402 with M rows and N columns.

The semiconductor substrate 404 can contain any suitable semiconductormaterial on which electric devices such as memory cell transistors canbe formed. Examples of semiconductor materials include silicon, galliumarsenide, indium phosphide, or the like. The isolation regions 406 cancontain any suitable dielectric material such as oxides. Examples ofoxides include silicon oxide, HTO, HDP oxide, or the like. In anotherembodiment, the isolation region 406 contains an oxide that is formedusing a Slot Plane Antenna (SPA) process.

The configuration and/or constituent of the gate structure 410 may varyand are not critical to the subject innovation. The gate structure 410may, for example, include a floating gate 412 and a select gate 414,separated by an interpoly dielectric layer 416. Examples of interpolylayers include an oxide/nitride/oxide tri-layer. The gate structure mayfurther contain any suitable layer. For example, the gate structurecontains a tunnel oxide 418 between the semiconductor substrate 404 andthe floating gate 412. In another embodiment, the gate structurecontains an insulating layer 420 such as an oxide layer.

Layers/components of the gate structure 410 can by formed by anysuitable technique. For example, the gate structure 410 can be formed byCVD, lithography, and etching techniques. Implant regions (e.g.,source/drain regions) 408 can be formed within the semiconductorsubstrate 404 by any suitable technique. For example, the implantedregion 408 is formed via implantation of one or more dopants such asN-type dopants (e.g., arsenic, phosphorous, antimony).

FIG. 5 illustrates forming layers of neutron-reactant material 500 overmemory cells 402 in the portions 400 a, 400 b. The layer 500 containsany suitable neutron-reactant material that can react with a neutron andemit one or more particles capable of changing the state of a memorycell 402. For example, the layer 500 contains any of theneutron-reactant materials of the neutron-reactant layer 318 asdescribed above in connection with FIG. 3. The layer 500 can be formedby any suitable technique. For example, the layer 500 is formed in thesame manner as described for forming the neutron-reactant layer 318 inconnection with FIG. 3. As shown in FIG. 5, the layer may be formed withan irregular surface. Reheating the layer can cause a reflow of theneutron-reactant material and can reduce the height of peaks and reducethe depths of valleys. Thus, an optional reheating act causing thereflow of the neutron-reactant material may be conducted.

FIG. 6 illustrates removing portions of the neutron-reactant material500, thereby forming contact openings 600 to active portions (e.g.,source/drain regions 408 and select gate 414) of the memory cell 402 andforming neutron-reactant layers 602. Portions of the neutron-reactantmaterial 500 can be removed by any suitable technique. For example, theportions are removed by chemical-mechanical polishing (CMP),lithography, and etching techniques.

The resultant neutron-reactant layers 602 can have any suitablethickness to facilitate detecting neutrons. The thickness may vary andis not critical to the subject innovation. The thickness of theneutron-reactant layers 602 may depend on, for example, on neutronenergy levels involved, desirable dimensions (e.g., thickness) of theneutron detector 400, the configuration and/or constituent of theneutron-reactant layer, the desired implementations and/or the neutrondetector 400 being fabricated, or the like. For example, the thicknessof the neutron-reactant layer is selected to allow penetration of someof the emitted particles, such as ⁴Alpha, into the underlying memorycell 402. In one embodiment, a thickness of a neutron-reactant layer 602is about 50 nm or more and about 1,000 nm or less. In anotherembodiment, a thickness of a neutron-reactant layer 602 is about 100 nmor more and about 700 nm or less. In yet another embodiment, a thicknessof a neutron-reactant layer 602 is about 200 nm or more and about 500 nmor less.

FIG. 7 illustrates forming a conductive component 700 (e.g., conductivecontact 702 and conductive layer 704) over the semiconductor substrate404. A conductive contact 702 can be formed in the contact opening 600to electrically contact active portions of the memory cell 402. Theconductive layer 702 can be formed over the substrate 404 toelectrically couple the conductive contacts 702. The conductivecomponent 700 can be formed by any suitable technique. For example, theconductive component 700 is formed by depositing a metal, such astungsten or aluminum, via CVD, physical vapor deposition (PVD) over thesemiconductor substrate 404 and removing unnecessary portions of themetal using, for example, photolithography and etching techniques.

FIG. 8 illustrates forming a gamma shield 800 over memory cells 402 inthe portion of a memory die 400 b, thereby forming a neutron detector400. The gamma shield 800 contains any suitable gamma shield materialthat can substantially prevent penetration of gamma rays into thecovered portion of the memory cells 402 but allow penetration ofneutrons into the memory cells 402. For example, the gamma shield 800contains any of the materials of the gamma shield 320 as described abovein connection with FIG. 3. The gamma shield 800 can be formed by anysuitable technique. For example, the gamma shield 800 is formed in thesame manner as described for forming the gamma shield 320 in connectionwith FIG. 3.

Although not shown, a passivation layer can be formed over the substrate404 to protect and/or cover the neutron detector 400. The passivationlayer can contain any suitable material such as oxides. For example, thepassivation layer contains silicon dioxide. Suitable thicknesses for thepassivation layer range from about 50 nm or more and about 500 nm orless.

FIGS. 9 to 11 diagrammatically illustrate how the state of memory cellscan change in the presence of a neutron field. Generally, neutrons aredetected by determining whether or not the state of a memory cell ischanged. FIG. 9 illustrates a portion of an exemplary neutron detector900. In FIG. 9, there are illustrated two memory transistors (e.g., aleft memory cell 902 and a right memory cell 904) having thereover aneutron-reactant layer 906, such as BPSG with a relatively highconcentration of ¹⁰Boron. The portion of a memory die 900 a of theneutron detector 900 does not include a gamma shield over the memorycell 902. The portion of another memory die 900 b of the neutrondetector 900 can further contain a gamma shield 908 over the memory cell904. In this example, the initial undisturbed states of the memory cells902, 904 are an on-state or a logical 1 state. Generally the logical 1state is associated with a negative charge on a floating gate 910.

FIG. 10 illustrates penetration of neutrons into the two memory cells902, 904 and penetration of gamma rays into the left memory cell 902that is not covered with a gamma shield. Gamma rays are blocked by thegamma shield 908 and substantially prevented from penetration into theunderlying memory cell 904 in the portion 900 b. FIG. 10 alsoillustrates a reaction occurring when a neutron penetrates theneutron-reactant layer 906 and reacts with a neutron-reactant materialsuch as a ¹⁰Boron atom in the neutron-reactant layer 906. The reactionof the neutron with the ¹⁰Boron atom generally produces a ⁷Lithiumparticle and a ⁴Alpha particle in accordance with the followingrelationship:

In+¹⁰B→⁷Li+⁴α

FIG. 11 illustrates changing states of the memory cells 902, 904 bypenetration of gamma ray and particles associated with neutrons. Whenthe alpha particle (⁴Alpha) and/or gamma ray pass through an inversionlayer (e.g., tunnel layer) beneath the floating layer 910, electronholes are produced and the charge in the channel region is sufficientlyreduced, thereby changing the state of the memory cells 902, 904 (e.g.,from the initial undisturbed state of logical 1 to the disturbed stateof logical 0). Since there is not a gamma shield over the left memorycell 902, in the portion 900 a gamma rays and particles associated withneutrons can penetrate the inversion layer of the left memory cell 902and change the status of the left memory cell 902. As a result, the leftmemory cell 902 can detect a neutron and gamma ray. To the contrary,since there is the gamma shield 908 over the right memory cell 904 inthe portion 900 b, gamma ray cannot substantially penetrate an inversionlayer of the right memory cell 904. As a result, the right memory cell904 can detect a neutron and discriminate against gamma rays.

Strength of a neutron field can be determined by reading a state of eachof the memory cells to determine a number of memory cells changing froman initial state to a disturbed state and using the number of memorycells having a disturbed state. The state of each of the memory cellsuncovered with the gamma shield and the state of each of the memorycells covered with the gamma shield can be read, separately. Thestrength of the neutron field can be determined by using a percentage ofthe memory cells having the disturbed state. When determining thestrength of the neutron field, the disturbed percentage of the memorycells uncovered with the gamma shield can be compared with the disturbedpercentage of the memory cells covered with the gamma shield.

FIG. 12 illustrates an exemplary methodology 1200 of forming a neutrondetector. At 1202, a neutron-reactant layer is formed over two or morememory dies. The memory dies can include a plurality of memory cells. At1204, one or more gamma shields are formed over at least a portion of oran entire of the two or more memory dies. In one embodiment, the two ormore memory dies contain a flash memory. In another embodiment, theneutron-reactant layer contains ¹⁰Boron. In yet another embodiment, thegamma shield contains lead. In still yet another embodiment, the gammashield covers about 5% of memory dies or more and about 95% of memorydies or less. The methodology can further involve forming a passivationlayer over the two or more memory dies.

Although not shown, the methodology of FIG. 12 may include any suitablememory die fabrication process. General examples of memory diefabrication processes include masking, patterning, etching,planarization, thermal oxidation, implant, annealing, thermal treatment,and deposition techniques normally used for making memory cells.

FIG. 13 illustrates an exemplary methodology 1300 of detecting strengthof a neutron field. At 1302, a neutron detector is provided. The neutrondetector can include two or more memory dies, neutron-reactant layersover the two or more memory dies, and one or more gamma shields over atleast a portion of or an entire of the two or more memory dies. Thememory die can include a plurality of memory cells.

At 1304, a state of each of the memory cells of the memory dies is readto determine a number of memory cells changing from an initial state toa disturbed state. At 1306, the strength of the neutron field isdetermined by using the number of memory cells having the disturbedstate.

Although not shown in FIG. 13, when reading the state of each of thememory cells, the state of each of the memory cells uncovered with thegamma shield and the state of each of the memory cells covered with thegamma shield are read separately. In another embodiment, whendetermining the strength of the neutron field, a percentage of thememory cells having the disturbed state is determined. In yet anotherembodiment, when determining the strength of the neutron field, thedisturbed percentage of the memory cells uncovered with the gamma shieldis compared with the disturbed percentage of the memory cells coveredwith the gamma shield.

What has been described above includes examples of the subjectinnovation. It is, of course, not possible to describe every conceivablecombination of components or methodologies for purposes of describingthe subject innovation, but one of ordinary skill in the art mayrecognize that many further combinations and permutations of the subjectinnovation are possible. Accordingly, the subject innovation is intendedto embrace all such alterations, modifications and variations that fallwithin the spirit and scope of the appended claims. Furthermore, to theextent that the term “includes” and “involves” are used in either thedetailed description or the claims, such terms are intended to beinclusive in a manner similar to the term “comprising” as “comprising”is interpreted when employed as a transitional word in a claim.

1. A neutron detector comprising: two or more memory dies comprising aplurality of memory cells, the memory dies comprising neutron-reactantlayers over the memory cells; and one or more gamma shields over atleast a portion of or an entire of the two or more memory dies, thegamma shield substantially preventing gamma rays from penetrating intothe memory die under the gamma shield.
 2. The neutron detector of claim1, wherein the two or more memory dies comprise a flash memory.
 3. Theneutron detector of claim 1, wherein the neutron-reactant layercomprises a borophosphosilicate glass that reacts with a neutron andemits a ⁴Alpha particle, and wherein the memory die uncovered with thegamma shield detects the ⁴Alpha particle and gamma rays, and the memorydie covered with the gamma shield detects the ⁴Alpha particle anddiscriminates against gamma rays.
 4. The neutron detector of claim 1,wherein the neutron-reactant layer comprises ¹⁰Boron that reacts with aneutron and emits a ⁴Alpha particle, and wherein the memory dieuncovered with the gamma shield detects the ⁴Alpha particle and gammarays, and the memory die covered with the gamma shield detects the⁴Alpha particle and discriminates against gamma rays.
 5. The neutrondetector of claim 4, wherein the neutron-reactant layer comprises about80 wt% or more and about 100 wt% or less of ¹⁰Boron in a total Boronconcentration.
 6. The neutron detector of claim 1, wherein the gammashield comprises lead.
 7. The neutron detector of claim 1, whereinthe-gamma-shield about 5% of memory dies or more and about 95% of memorydies or less is covered with the gamma shield.
 8. The neutron detectorof claim 1 further comprising a passivation layer over the two or morememory dies.
 9. A method of making a neutron detector, comprising:forming neutron-reactant layers over two or more memory dies; andforming one or more gamma shields over at least a portion of or anentire of the two or more memory dies, the gamma shield substantiallypreventing gamma rays from penetrating into the memory die under thegamma shield.
 10. The method of claim 9, wherein the two or more memorydies comprise a flash memory.
 11. The method of claim 9, wherein theneutron-reactant layer comprises a borophosphosilicate glass that reactswith a neutron and emits a ⁴Alpha particle, and wherein the memory dieuncovered with the gamma shield detects the ⁴Alpha particle and gammarays, and the memory die covered with the gamma shield detects the⁴Alpha particle and discriminates against gamma rays.
 12. The method ofclaim 9, wherein the neutron-reactant layer comprises ¹⁰Boron thatreacts with a neutron and emits a ⁴Alpha particle, and wherein thememory die uncovered with the gamma shield detects the ⁴Alpha particleand gamma rays, and the memory die covered with the gamma shield detectsthe ⁴Alpha particle and discriminates against gamma rays.
 13. The methodof claim 9, wherein the neutron-reactant layer comprises about 80 wt% ormore and about 100 wt% or less of ¹⁰Boron in a total Boronconcentration.
 14. The method of claim 9, wherein the gamma shieldcomprises lead.
 15. The method of claim 9, wherein the gamma shieldcovers about 5% of memory dies or more and about 95% of memory dies orless.
 16. The method of claim 9 further comprising forming a passivationlayer over the two or more memory dies.
 17. A method of detectingstrength of a neutron field, comprising: providing a neutron detectorcomprising two or more memory dies, neutron-reactant layers over the twoor more memory dies, and one or more gamma shields over at least aportion of or an entire of the two or more memory dies, the memory diecomprising a plurality of memory cells, the gamma shield substantiallypreventing gamma rays from penetrating into the memory die under thegamma shield; reading a state of each of the memory cells of the memorydies to determine a number of memory cells changing from an initialstate to a disturbed state; and determining the strength of the neutronfield by using the number of memory cells having the disturbed state.18. The method of claim 17, wherein reading the state of each of thememory cells comprises reading the state of each of the memory cellsuncovered with the gamma shield and reading the state of each of thememory cells covered with the gamma shield.
 19. The method of claim 17,wherein determining the strength of the neutron field comprisesdetermining a percentage of the memory cells having the disturbed state.20. The method of claim 17, wherein determining the strength of theneutron field comprises comparing the disturbed percentage of the memorycells uncovered with the gamma shield and the disturbed percentage ofthe memory cells covered with the gamma shield.